Light olefins serve as the building blocks for the production of numerous chemicals. Light olefins have traditionally been produced through the process of steam or catalytic cracking. The search for alternative materials for light olefin production has led to the use of oxygenates such as alcohols, and more particularly to the use of methanol, ethanol, and higher alcohols or their derivatives wherein these compounds are converted to light olefins. The alcohols may be produced by fermentation or from synthesis gas. Synthesis gas can be produced from natural gas, petroleum liquids, and carbonaceous materials including coal, recycled plastics, municipal wastes, or any organic material. Thus, alcohol and alcohol derivatives may provide non-petroleum based routes for the production of olefin and other related hydrocarbons.
Molecular sieves such as the microporous crystalline zeolite and non-zeolitic catalysts, particularly silicoaluminophosphates (SAPO), are known to promote the conversion of oxygenates to hydrocarbon mixtures. Numerous patents describe this process for various types of these catalysts: U.S. Pat. Nos. 3,928,483, 4,025,575, 4,052,479 (Chang et al.); U.S. Pat. No. 4,496,786 (Santilli et al.); U.S. Pat. No. 4,547,616 (Avidan et al.); U.S. Pat. No. 4,677,243 (Kaiser); U.S. Pat. No. 4,843,183 (Inui); U.S. Pat. No. 4,499,314 (Seddon et al.); U.S. Pat. No. 4,447,669 (Hamon et al.); U.S. Pat. No. 5,095,163 (Barger); U.S. Pat. No. 5,191,141 (Barger et al.); U.S. Pat. No. 5,126,308 (Barger et al.); U.S. Pat. No. 4,973,792 (Lewis et al.); and U.S. Pat. No. 4,861,938 (Lewis et al.). U.S. Pat. No. 4,861,938 and 4,677,242 particularly emphasize the use of a diluent combined with the feed to the reaction zone to maintain sufficient catalyst selectivity toward the production of light olefin products, particularly ethylene. The above U.S. patents are hereby incorporated by reference.
The product produced by the oxygenate conversion process is a light gas stream containing lighter components (e.g. hydrogen, nitrogen, etc.) methane, ethane and a substantial quantity of hydrocarbons of higher molecular weight, for example, propane, butane, pentane and often their unsaturated analogs. Separation of these components to recover ethylene requires a complex energy intensive scheme, thus creating a need for more efficient separation processes which yields higher recovery levels of ethylene. In typical ethylene plant recovery sections, where the ethylene production is based on the pyrolysis of naphtha or gas oil, the use of cryogenic processes utilizing the principle of gas expansion through a mechanical device to produce power while simultaneously extracting heat from the system have been employed. The use of such equipment varies depending upon the pressure of the product gas stream, the composition of the gas, and the desired end results. In the typical cryogenic expansion-type recovery processes used in the prior art, a gas stream is withdrawn from the pyrolysis furnace, compressed, and cooled. The cooling is accomplished by heat exchange with other streams of the process and/or external sources of cooling are employed such as refrigeration systems. As the product gas is cooled, liquids are condensed, collected, and separated so as to thereby obtain desired hydrocarbons. The high-pressure liquid stream so recovered is typically transferred to a demethanizer column after the pressure is adjusted to the operating pressure of the demethanizer. In such a fractionating column, the high-pressure liquid stream is fractionated to separate the residual methane and lighter components from the desired products of ethylene and heavier hydrocarbon components. In the ideal operation of such separation processes, the vapors, or light cut, leaving the process contain substantially all of the methane and lighter components found in the feed gas and substantially no ethylene and heavier hydrocarbon components remain. The bottom fraction, or heavy cut, leaving the demethanizer typically contains substantially all of the ethylene and heavier hydrocarbon components with very little methane or lighter components which are discharged in the fluid gas outlet from the demethanizer. A typical combined gas expansion and fractionation process for the separation of hydrocarbon gas stream comprising components ranging from nitrogen through C3-plus hydrocarbons into a methane and lighter stream and an ethylene and heavier stream is exemplified by U.S. Pat. No. 4,895,584. A typical ethylene separation section of an ethylene plant containing both cryogenic and fractionation steps to recover an ethylene product with a purity exceeding 99.5% ethylene is described in an article by V. Kaiser and M. Picciotti entitled, "Better Ethylene Separation Unit," appeared in Hydrocarbon Processing, November 1988, pages 57-61 and is herein incorporated by reference.
Pressure swing adsorption (PSA) provides an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbable gas can be an impurity which is removed from the less strongly adsorbable gas which is taken off as product; or, the more strongly adsorbable gas can be the desired product, which is separated from the less strongly adsorbable gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feedstream to produce a purified (99+%) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbable gases, such as ethylene from a feedstream to produce an ethylene-rich product.
In PSA, a multi-component gas is typically fed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, the feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more cocurrent depressurization steps wherein pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a countercurrent depressurization step wherein the pressure on the adsorption zone is further reduced by withdrawing desorbed gas countercurrently to the direction of the feedstream. Finally, the adsorption zone is purged and repressurized. The combined gas stream produced during the countercurrent depressurization step and the purge step is typically referred to as the tail gas stream. The final stage of repressurization is typically performed by introducing a slipstream of product gas comprising the lightest gas component produced during the adsorption step. This final stage of repressurization is often referred to as product repressurization. In multi-zone systems, there are typically additional steps, and those noted above may be done in stages. U.S. Pat. No. 3,176,444 issued to Kiyonaga, U.S. Pat. No. 3,986,849 issued to Fuderer et al., and U.S. Pat. No. 3,430,418 and U.S. Pat. No. 3,703,068 both issued to Wagner, among others, describe multi-zone, adiabatic PSA systems employing both cocurrent and countercurrent depressurization, and the disclosures of these patents are incorporated by reference in their entireties.
Various classes of adsorbents are known to be suitable for use in PSA systems, the selection of which is dependent upon the feedstream components and other factors generally known to those skilled in the art. In general, suitable adsorbents include molecular sieves, silica gel, activated carbon, and activated alumina. When PSA processes are used to purify hydrogen-containing streams, the hydrogen is essentially not adsorbed on the adsorbent. However, when purifying methane-containing streams, methane is often adsorbed on the adsorbent along with the impurity.
Numerous patents disclose the use of PSA in combination with fractionation to separate hydrogen and methane from heavier hydrocarbons. U.S. Pat. No. 5,245,099, which is hereby incorporated by reference, discloses a process for the concentration and recovery of ethylene and heavier components from a hydrocarbon feedstream. A PSA process is used to remove from hydrocarbon feedstream light cut comprising hydrogen, carbon monoxide, and methane and subsequently concentrate a heavy cut comprising the ethylene and heavy components in the PSA tail gas. In one aspect of the invention, an FCC off gas is separated into a light cut and a heavy cut and the heavy cut is routed to an ethylene plant.
U.S. Pat. No. 5,332,492 discloses a process for recovering hydrogen-rich gases and increasing the recovery of liquid hydrocarbon products from a hydrocarbon conversion zone effluent by the particular arrangement of refrigeration and PSA steps and two vapor-liquid, or flash, separation zones.
U.S. Pat. Nos. 5,365,011, 5,470,925, and 5,744,687 disclose a process for the integration of a PSA zone containing an adsorbent selective for the adsorption of ethylene and propylene from a catalytic cracking process at an adsorption temperature above 50.degree. C. to about 250.degree. C. The adsorbent is selected from the group consisting of zeolite 4A, zeolite 5A, zeolite 13X, and mixtures thereof.
The existing cryogenic and fractionation system in a typical ethylene recovery scheme can be employed in an oxygenate conversion process to recover ethylene, but the penalties of this operation are significant. The low level of light components such as hydrogen and methane relative to ethylene plant product compositions still are high enough to significantly raise the compression and refrigeration requirements in the recovery section of the oxygenate conversion plant for the incremental amount of ethylene recovered. Thus, recovering ethylene from oxygenate conversion effluent is an expensive and complex process involving extensive compression and fractionation to separate the light gases such as hydrogen and methane from the ethylene. Processes are sought which enable the concentration and recovery of the ethylene and heavier components from oxygenate conversion effluent without expensive compression and cryogenic separation steps to remove the lighter components.